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Journal Articles

Bromine-isotope selective ionization using field-free alignment of IBr isotopologues with a switched nanosecond laser pulse

Akagi, Hiroshi*; Kumada, Takayuki; Otobe, Tomohito*; Itakura, Ryuji*; Hasegawa, Hirokazu*; Oshima, Yasuhiro*

Chemistry Letters, 49(4), p.416 - 418, 2020/04

 Times Cited Count:0 Percentile:0(Chemistry, Multidisciplinary)

Journal Articles

Isotope-selective ionization utilizing field-free alignment of isotopologues using a switched nanosecond laser pulse

Akagi, Hiroshi*; Kumada, Takayuki; Otobe, Tomohito*; Itakura, Ryuji*; Hasegawa, Hirokazu*; Oshima, Yasuhiro*

Applied Physics B, 124(1), p.14_1 - 14_8, 2018/01

 Times Cited Count:1 Percentile:11.8(Optics)

We propose and numerically simulate a method of laser isotope separation based on field-free alignment of isotopologues, utilizing an intense switched nanosecond (ns) laser field which is slowly turned on and rapidly turned off at the peak with the falling time of 200 fs. The femtosecond (fs) laser induced alignment of molecules including a heavy atom is severely disturbed by ionization because of their small ionization potential. Our simulations for $$^I79$$Br and $$^I81$$Br isotopologues demonstrate that the switched ns laser field can make isotopologues well-aligned with the reduced ionization probability at the laser intensity which is an order-of-magnitude lower than a typical intensity for field-free alignment induced by a fs laser field.

Journal Articles

Plasma-mirror frequency-resolved optical gating for simultaneous retrieval of a chirped vacuum-ultraviolet waveform and time-dependent reflectivity

Itakura, Ryuji*; Kumada, Takayuki; Nakano, Motoyoshi*; Akagi, Hiroshi*

High Power Laser Science and Engineering, 4, p.e18_1 - e18_5, 2016/06

 Times Cited Count:3 Percentile:24.58(Optics)

We demonstrate that the methodology of frequency-resolved optical gating (FROG) is applicable to time-resolved reflection spectroscopy of a plasma mirror in the vacuum-ultraviolet (VUV) region. Our recent study has shown that a VUV waveform can be retrieved from a VUV reflection spectrogram of a plasma mirror formed on a fused silica (FS) surface by irradiation with an intense femtosecond laser pulse. Simultaneously, the increase in the reflectivity with respect to the Fresnel reflection of the unexcited FS surface can be obtained as a time-dependent reflectivity of the plasma mirror. In this study, we update the FROG analysis procedure using the least-square generalized projections algorithm. This procedure can reach convergence much faster than the previous one and has no aliasing problem. It is demonstrated that a significantly chirped VUV pulse as long as 1 ps can be precisely characterized.

Journal Articles

Correlation between photoeletron and photoion in ultrafast multichannel photoionization of Ar

Itakura, Ryuji; Fushitani, Mizuho*; Hishikawa, Akiyoshi*; Sako, Tokuei*

AIP Conference Proceedings 1702, p.090021_1 - 090021_4, 2015/12

 Times Cited Count:0 Percentile:0

We theoretically investigate coherent dynamics of ions created through ultrafast multichannel photoionization from a viewpoint of photoelectron-photoion correlation. The model calculation on single-photon ionization of Ar reveals that the coherent hole dynamics in Ar$$^+$$ associated with a superposition of the spin-orbit states $$^2P_J$$ (J = 3/2 and 1/2) can be identified by monitoring only the photoion created by a Fourier-transform limited extreme ultraviolet (EUV) pulse with the 9-fs pulse duration, while the coherence is lost by a chirped EUV pulse. It is demonstrated that by coincidence detection of the photoelectron and photoion the coherent hole dynamics can be extracted even in the case of ionization by a chirped EUV pulse with the sufficiently wide bandwidth.

Journal Articles

Non-thermal effects on femtosecond laser ablation of polymers extracted from the oscillation of time-resolved reflectivity

Kumada, Takayuki; Akagi, Hiroshi; Itakura, Ryuji; Otobe, Tomohito; Nishikino, Masaharu; Yokoyama, Atsushi

Applied Physics Letters, 106(22), p.221605_1 - 221605_5, 2015/06

 Times Cited Count:3 Percentile:17.22(Physics, Applied)

The dynamics of femtosecond laser ablation of transparent polymers were examined using time-resolved reflectivity. When these polymers were irradiated by a pump pulse with fluence above the ablation threshold of 0.8-2.0 J/cm$$^{2}$$, we observed the oscillation of the reflectivity caused by the interference between the reflected probe pulses from the sample surface and the thin layer due to the non-thermal photomechanical effects of spallation. As the fluence of the pump pulse increased, the separation velocity of the thin layer increased from 6 km/s to an asymptotic value of 11 km/s. It is suggested that the velocities are determined by shock-wave velocities of the photo-excited layer.

Journal Articles

Isotope-selective ionization utilizing field-free alignment of isotopologues with a train of femtosecond laser pulses

Akagi, Hiroshi; Kasajima, Tatsuya*; Kumada, Takayuki; Itakura, Ryuji; Yokoyama, Atsushi; Hasegawa, Hirokazu*; Oshima, Yasuhiro*

Physical Review A, 91(6), p.063416_1 - 063416_7, 2015/06

 Times Cited Count:7 Percentile:45.32(Optics)

We propose a strategy of isotope-selective ionization for a binary mixture of isotopologues of homonuclear diatomic molecules, utilizing field-free alignment with a train of femtosecond laser pulses. Field-free alignment can be achieved simultaneously for two isotopologues consisting of two atoms with the same atomic mass number $$alpha$$ or $$beta$$, utilizing a pulse train with their time interval of T$$_{com}$$ = $$beta$$ T($$alpha$$) = $$alpha$$ T($$beta$$), where T($$alpha$$) and T($$beta$$) are the rotational revival times of the isotopologues. We demonstrate experimentally that a train of four alignment pulses with their interval of T$$_{com}$$ ($$alpha$$ = 14, $$beta$$ = 15) creates transiently aligned $$^{14}$$N$$_{2}$$ and anti-aligned $$^{15}$$N$$_{2}$$ just before T$$_{com}$$/2 after the last pulse, and vice versa just after T$$_{com}$$/2. Highly isotope-selective N$$_{2}$$ ionization is achieved at these timings with another femtosecond laser pulse, which induces the non-resonant multiphoton ionization with the cross section remarkably depending on the angle between the molecular axis and the laser electric field direction. The ion yield ratio I($$^{15}$$N$$_{2}$$$$^{+}$$)/I($$^{14}$$N$$_{2}$$$$^{+}$$) ranges from 0.49 to 2.00, which is wider than the range obtained with single alignment pulse.

Journal Articles

Frequency-resolved optical gating for characterization of VUV pulses using ultrafast plasma mirror switching

Itakura, Ryuji; Kumada, Takayuki; Nakano, Motoyoshi*; Akagi, Hiroshi

Optics Express (Internet), 23(9), p.10914 - 10924, 2015/05

 Times Cited Count:15 Percentile:70.58(Optics)

We propose and experimentally demonstrate a method for characterizing vacuum ultraviolet (VUV) pulses based on time-resolved reflection spectroscopy of fused silica pumped by an intense laser pulse. Plasma mirror reflection is used as an ultrafast optical switch, which enables us to measure frequency-resolved optical gating (FROG) traces. The VUV temporal waveform can be retrieved from the measured FROG trace using principal component generalized projections algorithm with modification. The temporal profile of the plasma mirror reflectivity is also extracted simultaneously.

Journal Articles

Photoelectron emission and intramolecular excitation of molecules in intense laser fields

Itakura, Ryuji

Kyo Koshiba Kagaku No Tenkai CSJ Karento Rebyu, p.52 - 57, 2015/03

In dissociative ionization of molecules induced by intense laser fields, a variety of reaction channels appear resulting from the combination of electron emission, electron excitation in molecules and so on. It is important to separate (1) photoelectron emission and (2) subsequent electronic excitation in molecules. We review the recent experimental results obtained by the photoelectron-photoion coincidence method.

Journal Articles

Preface to topical papers on frontier of pulsed electron beam for research on photoexcited dynamics

Itakura, Ryuji

Reza Kenkyu, 43(3), p.136 - 137, 2015/03

Recently, technique of pulsed electron beam generation combined with ultrafast laser technology has been developed for realization of time-resolved measurements. A variety of methods such as an electron gun with a photo-cathode and an optical gating of a CW electron beam are proposed and demonstrated. We will compiles topical papers on frontier of pulsed electron beam for research on photoexcited dynamics in one volume of "The Review of Laser Engineering". This paper is a preface for describing the aim and contents of this special volume.

Journal Articles

Electron and ion coincidence momentum imaging of multichannel dissociative ionization of ethanol in intense laser fields

Itakura, Ryuji; Hosaka, Koichi*; Yokoyama, Atsushi; Ikuta, Tomoya*; Kannari, Fumihiko*; Yamanouchi, Kaoru*

Progress in Ultrafast Intense Laser Science XI; Springer Series in Chemical Physics, Vol.109, p.23 - 42, 2015/00

We investigate the multichannel dissociative ionization of ethanol in intense laser fields by the photoelectron-photoion coincidence momentum imaging and identify separately the ionization and subsequent electronic excitation in ethanol. From the energy correlation between a photoelectron and a fragment ion, we reveal the amount of the internal energy gained by ethanol cations from the laser field varies depending on the respective ionization and electronic excitation pathways.

Journal Articles

Photoelectron-photoion correlation in ultrafast multichannel photoionization of Ar

Itakura, Ryuji; Fushitani, Mizuho*; Hishikawa, Akiyoshi*; Sako, Tokuei*

Journal of Physics B; Atomic, Molecular and Optical Physics, 47(19), p.195602_1 - 195602_9, 2014/10

 Times Cited Count:2 Percentile:15.6(Optics)

Ultrafast multichannel single-photon ionization of Ar is investigated theoretically by a model calculation incorporating correlation between the photoelectron and photoion. It is shown that the coherent hole dynamics in Ar$$^+$$ associated with a superposition of the spin-orbit states can be observed by using a Fourier-transform limited laser pulse for ionization, while the coherence is degraded or lost by chirping a laser pulse. We demonstrate that the coherent hole dynamics can be retrieved even in the case of a chirped laser pulse, by labeling the photoion dynamics with the counterpart photoelectron energy.

Journal Articles

Femtosecond laser ablation dynamics of fused silica extracted from oscillation of time-resolved reflectivity

Kumada, Takayuki; Akagi, Hiroshi; Itakura, Ryuji; Otobe, Tomohito; Yokoyama, Atsushi

Journal of Applied Physics, 115(10), p.103504_1 - 103504_9, 2014/03

 Times Cited Count:16 Percentile:62.88(Physics, Applied)

Femtosecond laser ablation dynamics of fused silica is examined via time-resolved reflectivity measurements. After optical breakdown was caused by irradiation of a pump pulse, the reflectivity oscillated with a period of 63 ps for a wavelength 795 nm. The period was reduced by half for 398 nm. We ascribe the oscillation to the interference between the probe pulses reflected from the front and rear surfaces of the photo-excited molten fused silica layer.

Journal Articles

Correlation between a photoelectron and a fragment ion in dissociative ionization of ethanol in intense near-infrared laser fields

Hosaka, Koichi*; Yokoyama, Atsushi; Yamanouchi, Kaoru*; Itakura, Ryuji

Journal of Chemical Physics, 138(20), p.204301_1 - 204301_9, 2013/05

 Times Cited Count:12 Percentile:44.57(Chemistry, Physical)

Dissociative ionization of ethanol (C$$_{2}$$H$$_{5}$$OH) induced by an intense near-infrared laser pulse are investigated using photoelectron-photoion coincidence method. It is shown that both the electronic ground state and the first electronically excited state of C$$_{2}$$H$$_{5}$$OH$$^{+}$$ are produced at the moment of photoelectron emission. From the observed correlation between the electronic states of C$$_{2}$$H$$_{5}$$OH$$^{+}$$ prepared at the moment of photoelectron emission and the kinetic energy release of the fragment ions, it is revealed that C$$_{2}$$H$$_{5}$$OH$$^{+}$$ prepared in the electronic ground state at the photoelectron emission gains larger internal energy in the end than that prepared in the electronically excited state. The averaged internal energy of C$$_{2}$$H$$_{5}$$OH$$^{+}$$ just before the dissociation is found to increase when the laser field intensity increases from 9 to 23 TW/cm$$^{2}$$. And when the laser pulse duration increases from 35 to 800 fs.

Journal Articles

Isotope-selective ionization utilizing molecular alignment and non-resonant multiphoton ionization

Akagi, Hiroshi; Kasajima, Tatsuya; Kumada, Takayuki; Itakura, Ryuji; Yokoyama, Atsushi; Hasegawa, Hirokazu*; Oshima, Yasuhiro*

Applied Physics B, 109(1), p.75 - 80, 2012/10

 Times Cited Count:14 Percentile:61.35(Optics)

We demonstrate a laser nitrogen isotope separation, which is based on field-free alignment and angular dependent ionization of $$^{14}$$N$$_{2}$$ and $$^{15}$$N$$_{2}$$ isotopomers. A linearly-polarized short laser pulse ($$lambda$$$$sim$$795 nm, $$Delta$$$$tau$$$$sim$$60 fs) creates rotational wave packets in the isotopomers, which periodically revive with different revival times as a result of different moments of inertia. Another linearly-polarized short laser pulse ($$lambda$$$$sim$$795 nm, $$Delta$$$$tau$$$$sim$$60 fs) ionizes one of the isotopomers selectively as a result of their different angular distributions. In the present experiments, the ion yield ratio $$R$$ [= $$I$$($$^{15}$$N$$_{2}$$$$^{+}$$)/$$I$$($$^{14}$$N$$_{2}$$$$^{+}$$)] can be changed in the range from 0.85 to 1.22, depending on the time delay between the two laser pulses.

Journal Articles

Preface to topical papers on frontier of ultrafast molecular spectroscopy

Itakura, Ryuji

Reza Kenkyu, 40(10), p.737 - 738, 2012/10

We edit a special issue of "The Review of Laser Engineering", which is a monthly journal of the laser society of Japan. In this review, I describe the aim of this special issue and the current status of the field of "Ultrafast molecular spectroscopy" in connection with 6 invited papers published in this volume.

Journal Articles

Separation of ionization and subsequent electronic excitation for formation of electronically excited ethanol cation in intense laser fields

Ikuta, Tomoya*; Hosaka, Koichi*; Akagi, Hiroshi; Yokoyama, Atsushi; Yamanouchi, Kaoru*; Kannari, Fumihiko*; Itakura, Ryuji

Journal of Physics B; Atomic, Molecular and Optical Physics, 44(19), p.191002_1 - 191002_5, 2011/10

 Times Cited Count:8 Percentile:39.78(Optics)

Ionization and subsequent electronic excitation occurring within the same laser pulse (400 nm, 96 fs, 1.3$$sim$$18 TW/cm$$^{-2}$$) are separately investigated by measuring in coincidence an electron and a product ion produced from C$$_{2}$$H$$_{5}$$OH. We reveal that the nascent population in the electronically excited C$$_{2}$$H$$_{5}$$OH$$^{+}$$ prepared by the ionization decreases as the laser intensity increases, while the subsequent electronic excitation is enhanced through the resonant electronic transitions. Ionization and electronic excitation mechanisms are described based on the electronic state distributions of C$$_{2}$$H$$_{5}$$OH$$^{+}$$.

Journal Articles

Reconstruction of attosecond pulses using a two-color pumping

Chen, J.*; Itakura, Ryuji; Nakajima, Takashi*

Journal of the Optical Society of America B, 28(9), p.2195 - 2199, 2011/09

 Times Cited Count:2 Percentile:14.42(Optics)

We propose a two-color pumping scheme to characterize attosecond extreme ultraviolet (XUV) pulses. The fundamental and its second harmonic of a fs Ti:Sapphire laser create a coherent superposition of the 4$$p$$ and 5$$p$$ states of K, and we retrieve the spectral phase of the XUV pulse from the phase offset of the photoelectron signal as a function of time delay after the pump pulse. The scheme is technically simple and efficient to characterize 100 attosecond pulses.

Journal Articles

Study on material science by using an intense laser

Akagi, Hiroshi; Itakura, Ryuji; Otobe, Tomohito; Kumada, Takayuki; Tsubouchi, Masaaki; Yokoyama, Atsushi

JAEA-Conf 2011-001, p.40 - 43, 2011/03

no abstracts in English

Journal Articles

Coherent correlation between nonadiabatic rotational excitation and angle-dependent ionization of NO in intense laser fields

Itakura, Ryuji; Hasegawa, Hirokazu*; Kurosaki, Yuzuru; Yokoyama, Atsushi; Oshima, Yasuhiro*

Journal of Physical Chemistry A, 114(42), p.11202 - 11209, 2010/07

 Times Cited Count:10 Percentile:35.04(Chemistry, Physical)

When neutral NO molecules are partly ionized in intense laser fields ($$I$$$$_{0}$$ $$>$$ 35 TW/cm$$^{2}$$), a hole in the rotational wave packet of the remaining neutral NO is created by the ionization, whose rate depends on the alignment angle of the molecular axis with respect to the laser polarization direction. Rotational state distributions of NO are experimentally observed and then the characteristic feature that the population at higher $$J$$ levels is increased by the ionization can be identified. Numerical calculation for solving time-dependent rotational Schrodinger equations including the effect of the ionization is carried out. The numerical results suggest that NO molecules aligned perpendicular to the laser polarization direction are dominantly ionized at the peak intensity of $$I$$$$_{0}$$ = 42 TW/cm$$^{2}$$, where the multiphoton ionization is preferred rather than the tunneling ionization.

Journal Articles

Selective bond breaking in dissociative ionization of ethanol induced by tailored intense laser fields

Itakura, Ryuji; Yamanouchi, Kaoru*; Kannari, Fumihiko*

Advances in Multi-Photon Processes and Spectroscopy, Vol.19, p.93 - 115, 2010/03

It is demonstrated that the branching ratio between the C-C and C-O bond breaking processes in the dissociative ionization of ethanol is controlled by tailoring the intense laser pulse shape. Through systematic investigation of the dependence of the parameters of the laser pulses on the yield ratio between the two dissociation pathways, mechanisms of bifurcating into the two pathways are discussed. The important key factors for determining the yield ratio were found to be the pulse duration and wavelength. It was proposed that the nonadiabatic electronic excitation is efficiently induced by the wave packet evolution within the laser pulse duration.

133 (Records 1-20 displayed on this page)